Method for wavelength assignment in a WDM network

ABSTRACT

In accordance with the present disclosure, a method of configuring a wavelength division multiplexed (WDM) network is presented. The WDM network includes circuits that carry optical signals, with each signal corresponding to a wavelength. The WDM network includes nodes, with links connecting the nodes to one another. Each circuit includes at least one link and at least one node. The method comprises assigning each of the circuits to an optical signal, based on first and second criteria, and configuring the nodes based on the assignment.

BACKGROUND

In a wavelength division multiplexed (WDM) network, optical signals maybe switched from one optical communication link to another. In order toachieve such switching, the optical signals are often received from afirst optical link at a node and converted to electrical signals. Theelectrical signals are then used to generate further optical signals,which are supplied to a second optical link connected to the node.

An alternative approach involves all-optical switching, whereby opticalsignals are received from one or more links at a node, and directed tocorresponding output links at the node without optical-to-electricalconversion. Because there is no optical-to-electrical conversion at thenode, it may be difficult to convert a signal on a circuit to adifferent wavelength at the node. Accordingly, a circuit must have thesame wavelength assigned along each link in its path. In addition, anycircuits traversing a particular link in the network must each beassigned a different wavelength. Moreover, wavelength availability maydepend on factors other than the above-noted constraints. For example,certain transmitters may be less expensive than others.

Accordingly, there is a need for an optimal wavelength assignment methodthat takes into account multiple parameters.

SUMMARY

In accordance with the present disclosure, a method of configuring awavelength division multiplexed (WDM) network is presented. The WDMnetwork includes circuits that carry optical signals, with each signalcorresponding to a wavelength. The WDM network includes nodes, withlinks connecting the nodes to one another. Each circuit includes atleast one link and at least one node. The method comprises assigningeach of the circuits to an optical signal, based on first and secondcriteria, and configuring the nodes based on the assignment.

In accordance with the present disclosure, another method of configuringa WDM network is presented. The WDM network includes circuits that carryoptical signals, with each signal corresponding to a wavelength. The WDMnetwork includes nodes, with links connecting the nodes to one another.Each circuit includes at least one link and at least one node. Themethod comprises generating a conflict graph that includes vertices,with edges connecting the vertices; associating each circuit with avertex; assigning a subgroup of optical signals to a vertex;prioritizing each optical signal within the subgroup; assigning eachcircuit to an optical signal based on the two previous steps; andconfiguring the nodes based on the assignment.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a WDM network including optically switched nodes;

FIG. 2 is a diagram of a conflict graph consistent with an aspect of thepresent disclosure; and

FIG. 3 is illustrates an embodiment of the steps in configuring a WDMnetwork.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of theinvention, an example of which is illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Consistent with the present disclosure, a method is provided forconfiguring a WDM network by assigning optical signals to circuits inthe network based on first and second criteria and then configuring thenodes based on that assignment. In an initial step, the WDM network maybe modeled with a diagram displaying the nodes, circuits, links, andavailable wavelengths for each link in the WDM network. Then, in asubsequent step, a conflict graph is created based on the WDM networkthat includes vertices corresponding to circuits in the WDM network,edges corresponding to shared links in the WDM network, and wavelengthlists corresponding to each vertex. The wavelength lists in the conflictgraph should be sorted based on the priority of the wavelengths in thenetwork. In a later step, an algorithm, such as a greedy heuristicalgorithm, may be applied to the conflict graph in order to assignwavelengths to each circuit in the network. The nodes are thenconfigured within the network based on the assignment of wavelengthsfrom the third step.

FIG. 1 illustrates a diagram of an exemplary WDM network 100 includingseveral nodes 110-115. Each node is connected to at least one other nodeby one of links 120-124. For example, node 110 is connected to node 111by link 120, and node 111 is connected to node 112 by link 121. Inaddition, node 111 is connected to node 114 by link 122, and node 114 isconnected to node 113 by link 123. Further, node 114 is connected tonode 115 by link 124.

Typically, there are a limited number of wavelengths on each link thatcan be allocated to a given optical signal transmitted on a circuit.Although networking equipment can be designed to operate with a widerange of wavelengths, the wavelengths actually available in a linkwithin a network may be constrained to a subset of the set ofwavelengths supported by the equipment due to such things as the typeand quality of the deployed optical fibers, available amplificationsites, and the types of amplifiers used. In addition, when wavelengthsneed to be allocated to a set of circuits, the network may be alreadydeployed and some wavelengths may already be allocated to circuits thatwere previously provisioned.

In WDM network 100, the wavelengths available for allocation on link 120include wavelengths λ₃, λ₆, λ₇, and λ₉, and the wavelengths availablefor allocation on link 121 include wavelengths λ₃, λ₄, λ₆, λ₈, and λ₉.In addition, the wavelengths available for allocation on link 122include wavelengths λ₃, λ₄, λ₆, λ₈, and λ₉, and the wavelengthsavailable for allocation on link 123 include wavelengths λ₃, λ₄, λ₅, λ₆,and λ₈. Further, the wavelengths available for allocation on link 124include wavelengths λ₄, λ₅, λ₈, λ₉, and λ₁₁.

Within WDM network 100, several circuits C₁-C₇ are illustrated, witheach circuit including one or more links 120-124: circuit C₁ includeslinks 120 and 121; circuit C₂ includes links 121 and 122; circuit C₃includes links 121, 122, and 124; circuit C₄ includes links 122 and 123;circuit C₅ includes links 123 and 124; circuit C₆ includes links 122 and124; and circuit C₇ includes links 120 and 121.

FIG. 2 illustrates an exemplary conflict graph 200 that may be used tomodel WDM network 100 from FIG. 1 and to aid in the determination ofwhich available wavelength should be assigned to each circuit C₁-C₇ fromFIG. 1. Conflict graph 200 includes vertices 211-217 that representcircuits C₁-C₇ from FIG. 1, respectively. Wavelength lists 221-227 aredisplayed next to vertices 211-217. Each wavelength list 221-227corresponds to the wavelengths available for each circuit C₁-C₇,respectively. The wavelengths available for a given circuit are thosewavelengths that are available on all of the links included within acircuit. For example, for circuit C₁, which traverses links 120 and 121,the wavelengths that are available are wavelengths λ₃, λ₆, and λ₉,because those are the wavelengths that are available on both links 120and 121.

The wavelength lists 221-227 are each sorted based on the priorityassociated with each wavelength in the network. A network designer mayprioritize certain wavelengths if, for example, the designer has spareline system equipment operating in those wavelengths that could be usedin an upcoming network buildout or if there is a lower expense forequipment supporting certain wavelengths. Similarly, an equipmentmanufacturer designing a network for a customer may have a preferencefor equipment supporting certain wavelengths that it already has instock, rather than for equipment supporting other wavelengths that itdoes not have in stock or that has yet to be manufactured. In theexample shown in FIG. 2, the wavelength lists 221-227 were sorted basedon the following priority (those listed first having the higherpriority): λ₉, λ₈, λ₆, λ₄, λ₅, λ₃.

In the conflict graph 200, edges 230-1 through 230-14 are shownconnecting vertices 211-217 when the corresponding circuits C₁-C₇ shareat least one link. For example, circuit C₁ shares at least one link withcircuits C₂, C₃, and C₇, so edges 230-1, 230-2, and 230-3 are drawnconnecting vertex 211 with vertices 212, 213, and 217 (corresponding tocircuits C₁, C₂, C₃, and C₇). In assigning a wavelength to a vertex (andits corresponding circuit), the wavelength is chosen from itscorresponding list of available wavelengths, provided that the samewavelength is not assigned to vertices connected by an edge.

After a conflict graph has been created, different known algorithms,such as exhaustive enumeration and greedy heuristics can be used todetermine the assignment of wavelengths. One advantage of using anexhaustive enumeration algorithm is that if there are multiple solutionsto the assignment of wavelengths for a conflict graph, then thisalgorithm will determine all such solutions. One disadvantage of usingthe exhaustive enumeration algorithm is that this algorithm potentiallyrequires more computation than the greedy heuristics algorithm. Anexample of a greedy heuristic algorithm is a “Largest First, First Fit(LF+FF)” algorithm in which the vertices of the conflict graph are firstsorted in a “Largest First” list in descending order according to thenumber of edges connected to each vertex. To the extent that some of thevertices have the same number of edges, the vertices can be furthersorted in descending order in the “Largest First” list according to thelength of their corresponding wavelength lists (with shorter wavelengthlists receiving higher priority), or they can be further sortedaccording to the “rarity” of the wavelengths in the wavelength lists(with the set of “rarer” wavelengths receiving higher priority). Afterthe ordering is defined in the “Largest Fit” list, a wavelength isassigned to each vertex by using “First Fit.” “First Fit” means that,for a given vertex, a “fit” is attempted for each wavelength in itswavelength list in the order that it appears in the list until a“fitting” wavelength is found (i.e., a wavelength that is not used byany of the vertices that are connected by an edge to the given vertex).Once a “fitting” wavelength is found, it is assigned to the vertex.

By way of example, to create the “Largest First” list for the conflictgraph 200 in FIG. 2, the number of edges 230-1 through 230-14 connectedto each vertex must be determined. As can be seen in FIG. 2, vertex 213has 6 connected edges, vertex 212 has 5 connected edges, vertices 214and 216 each have 4 connected edges, and vertices 211, 215, and 217 eachhave 3 connected edges. Based solely on the number of edges, an initial“Largest Fit” ordering could be: 213, 212, 214, 216, 211, 215, and 217.Because the foregoing list includes some vertices with the same numberof edges, the list may be further sorted based on the length ofcorresponding wavelength lists. While vertices 214 and 216 have the samenumber of edges, the wavelength list 224 corresponding to vertex 214contains 4 wavelengths, while the wavelength list 226 corresponding tovertex 216 contains only 3 wavelengths. Due to the lower number ofwavelengths contained in the wavelength list corresponding to vertex216, vertex 216 may be ranked higher than vertex 214. Vertices 211, 215,and 217 all have the same number of edges and the same number ofwavelengths in their corresponding wavelength lists. These vertices canthus be sorted according to the “rarity” of the wavelengths in theircorresponding wavelength lists (with the “rarer” wavelength listsreceiving higher priority). Accordingly, for example, in the wavelengthlist corresponding to vertex 215, which includes wavelengths λ₈, λ₄, andλ₅, wavelengths λ₈ and λ₄ each appear 5 times in the conflict graph 200,and wavelength 5 appears once, for a total “rarity” score of 11 (5+5+1).The wavelength lists corresponding to both vertex 211 and 217 includeswavelengths λ₉, λ₆, and λ₃. Thus, wavelengths λ₆ and λ₃ each appear 4times in the conflict graph 200, and wavelength λ₉ appears 5 times, fora total “rarity” score of 13 (4+4+5). Because the “rarer” wavelengthlist receives higher priority (the list with the lower “rarity” score),vertex 215 is ranked higher than vertices 211 and 217. Based on theforegoing, the “Largest Fit” list can be ordered as follows (alsoshowing the corresponding wavelength lists):

Vertex 213: λ₉, λ₈, λ₄

Vertex 212: λ₉, λ₈, λ₆, λ₄, λ₃

Vertex 216: λ₉, λ₈, λ₄

Vertex 214: λ₈, λ₆, λ₄, λ₃

Vertex 215: λ₈, λ₅, λ₄

Vertex 211: λ₉, λ₆, λ₃

Vertex 217: λ₉, λ₆, λ₃

Once the “Largest Fit” list is created, the “First Fit” algorithm can beapplied to the list. Beginning with vertex 213, the first wavelength inthe list (λ₉) can be assigned, because no other wavelengths have yetbeen assigned (so nothing can conflict with this assignment). For vertex212, the first wavelength in the list (λ₉) cannot be assigned, becausevertex 212 shares an edge 230-4 with vertex 213, and vertex 213 isalready using wavelength λ₉; however, the second wavelength in the list(λ₈) can be assigned. For vertex 216, neither the first nor secondwavelength in the list (λ₉, λ₈) can be assigned, because they arealready assigned to vertices 213 and 212, which both share an edge withvertex 216; however, the last wavelength in the list (λ₄) can beassigned. For vertex 214, the first wavelength in the list (λ₈) can notbe assigned, because vertex 214 shares an edge 230-5 with vertex 212,and vertex 212 is already using wavelength λ₈; however, the secondwavelength in the list (λ₆) can be assigned. For vertex 215, the firstwavelength in the list (λ₈) can be assigned, because even though vertex212 was already assigned wavelength λ₈, vertex 215 does not share anedge with vertex 212. For vertex 211, the first wavelength in the list(λ₉) cannot be assigned, because vertex 211 shares an edge 230-2 withvertex 213, and vertex 213 is already using wavelength λ₉; however, thesecond wavelength in the list (λ₆) can be assigned. For vertex 217, thefirst wavelength in the list (λ₉) can not be assigned, because vertex217 shares an edge 230-8 with vertex 213, and vertex 213 is alreadyusing wavelength λ g; the second wavelength in the list (λ₆) can not beassigned, because vertex 217 shares an edge 230-3 with vertex 211, andvertex 211 is already using wavelength 6; however, the third wavelengthin the list (λ₃) can be assigned. Thus, of the following wavelengthassignments can be obtained in the example shown in FIG. 2:

Vertex 213: λ₉

Vertex 212: λ₈

Vertex 216: λ₄

Vertex 214: λ₆

Vertex 215: λ₈

Vertex 211: λ₆

Vertex 217: λ₃

After determining the wavelength assignment for each circuit in thenetwork as described above, the nodes within the network are thenconfigured based on that assignment.

Flowchart 300 in FIG. 3 shows the steps included in a method ofconfiguring a WDM network consistent with the present disclosure. In afirst step 310, a diagram of a WDM network 100 may be created. Thediagram preferably includes the nodes, circuits, links, and availablewavelengths for each link in the WDM network. In a second step 320, aconflict graph 200 is created. The conflict graph preferably includevertices corresponding to the circuits in the WDM network, edgescorresponding to shared links in the WDM network, and wavelength listscorresponding to each vertex. The wavelength lists are sorted based onthe priority of the wavelengths in the network. In a third step 330, analgorithm, such as a greedy heuristic algorithm, is applied in order toassign wavelengths to each circuit in the WDM network. One example of agreedy heuristic that can be used is a modified “Largest First, FirstFit” algorithm. In a fourth step 340, the nodes within the network areconfigured based on the assignment of the wavelengths to each circuit inthe network.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A method of configuring a wavelength division multiplexed (WDM)network, the WDM network including a plurality of circuits, each ofwhich carrying one of a plurality of optical signals, each of theplurality of optical signals having a corresponding one of a pluralityof wavelengths, the WDM network including a plurality of nodes and aplurality of links connecting the plurality of nodes to one another,each of the plurality of circuits including at least one of theplurality of links and at least one of the plurality of nodes, themethod comprising: generating a conflict graph, the conflict graphincluding a plurality of vertices and a plurality of edges connectingthe plurality of vertices; associating each of the plurality of circuitswith a corresponding one of the plurality of the plurality of vertices;assigning each of a plurality of subgroups of the plurality of opticalsignals to a corresponding one of the plurality of vertices;prioritizing each optical signal within each of the plurality ofsubgroups of optical signals; assigning each of the plurality ofcircuits one of the plurality of optical signals, based on said step ofprioritizing said each optical signal within each of the plurality ofsubgroups and said step of assigning said each of the plurality ofsubgroups; and configuring the plurality of nodes based on the assigningeach of the plurality of circuits one of the plurality of opticalsignals.
 2. A method in accordance with claim 1, wherein the assigningsaid each of the plurality of subgroups of the plurality of opticalsignals is based on a first criteria, and the assigning said each of theplurality of circuits is based on a second criteria.
 3. A method inaccordance with claim 2, wherein the first criteria includes anavailability of each of the plurality of optical signals and the secondcriteria includes a priority associated with each of the plurality ofwavelengths, the priority being based on at least one of expense ofequipment supporting each of the plurality of wavelengths and physicalproperties associated with each of the plurality of links.
 4. A methodin accordance with claim 1, wherein each of the plurality of linksincludes an optical fiber.
 5. A method in accordance with claim 1,further including ranking each of the plurality of circuits inaccordance with a greedy heuristic algorithm.
 6. A method in accordancewith claim 5, wherein the greedy heuristic algorithm includes a “LargestFirst, First Fit” algorithm.
 7. A method in accordance with claim 1,wherein each of the plurality of nodes includes an optical switch.
 8. Amethod in accordance with claim 7, wherein the optical switch isconfigured to pass light without an optical-to-electrical conversion. 9.A method in accordance with claim 1, wherein the conflict graph includesa plurality of edges, each of the plurality of edges linking pairs ofthe plurality of vertices, each of said pairs corresponding to pairs ofthe plurality of circuits that share one of the plurality of links. 10.A method in accordance with claim 1, further including ranking each ofthe plurality of circuits in accordance with an exhaustive enumerationalgorithm.